1. Field of the Invention
The present invention relates to a semiconductor device using a semiconductor element. In particular, the present invention relates to a semiconductor device including a memory device using a semiconductor element. Note that the semiconductor device in this specification indicates all the devices that operate by utilizing semiconductor characteristics.
2. Description of the Related Art
Memory devices using semiconductor elements are broadly classified into two categories: a volatile device that loses stored data when power supply stops, and a non-volatile device that retains stored data even when power is not supplied.
A typical example of a volatile memory device is a DRAM (dynamic random access memory). A DRAM stores data in such a manner that a transistor included in a memory cell is selected and electric charge is stored in a capacitor.
When data is read from a DRAM, electric charge in a capacitor is lost on the above principle; thus, another write operation is necessary every time data is read. Further, a transistor included in a memory cell has leakage current and electric charge flows into or out of a capacitor even when the transistor is not selected, so that the data retention time is short. For that reason, another write operation (refresh operation) is necessary at predetermined intervals, and it is difficult to sufficiently reduce power consumption. Furthermore, since stored data is lost when power supply stops, an additional memory device using a magnetic material or an optical material is needed in order to hold the data for a long time.
Another example of a volatile memory device is an SRAM (static random access memory). An SRAM retains data by using a circuit such as a flip-flop and thus does not need a refresh operation. This means that an SRAM has an advantage over a DRAM. However, cost per storage capacity is increased because of the use of a flip-flop or the like. Moreover, as in a DRAM, stored data in an SRAM is lost when power supply stops.
A typical example of a non-volatile memory device is a flash memory. A flash memory includes a floating gate between a gate electrode and a channel region in a transistor and stores data by holding electric charge in the floating gate. A flash memory, therefore, has advantages in that the data retention time is extremely long (almost permanent) and a refresh operation which is necessary in a volatile memory device is not needed (e.g., see Patent Document 1).
However, a gate insulating film included in a memory cell deteriorates by tunneling current generated in writing, so that the memory cell stops its function after a predetermined number of write operations. In order to reduce adverse effects of this problem, a method for equalizing the number of write operations for memory cells is employed, for example, in which case a complicated peripheral circuit is needed. Moreover, employing such a method does not solve the fundamental problem of lifetime. In other words, a flash memory is not suitable for applications in which data is frequently rewritten.
In addition, high voltage is necessary for injecting electric charge in the floating gate or removing the electric charge, and a circuit for generating high voltage is also necessary. Further, it takes a relatively long time to inject or remove electric charge, and it is not easy to perform writing and erasing at higher speed.